The present invention relates to electromagnetic apparatus for producing linear motion. A number of designs of linear electromagnetic actuator, sometimes termed a linear motor, have been produced. Various linear motor configurations are described in WO93/01646. The devices to which this prior art relates have axial symmetry and are formed as piston-in-cylinder machines. The principal advantage of axial symmetry lies in the fact that the strong attractive forces between the magnetic elements of the stator and the magnetic elements of the armature are balanced about a central axis, so that the bearings of the machine do not need to withstand attractive forces.
A further advantage of the axially-symmetric construction is that the magnetic fields of the machine, whether generated as a result of electrical currents in conductors or produced by permanent magnets, can be contained within an outer steel case of the actuator. However they are generated, the magnetic fields can intersect the electrical coils of the machine with a high degree of efficiency.
Yet another advantage of the axially-symmetric construction is that, by utilising a cylindrical stator it is possible to form a sliding seal between the stator and the armature, which then allows of the possibility of creating a gas spring between the stator and armature by closing one end of the stator. Such an arrangement is described in more detail in the Applicant""s international application PCT/GB98/02823.
All these known linear actuators are constructed and designed to apply a force between machine components connected or mechanically linked to the stator and armature, for which purpose these have a configuration which permits such connection. However, there are certain mechanical systems in which it is not required to connect a moving element physically to a drive element. In such circumstances the armature of the actuator does not have to have a continuous direct connection to the external environment. For example, it may be required for an element to be repeatedly driven to reciprocate along a guide member so as to deliver energy at the ends of its stroke when it changes direction. A road breaker or so-called xe2x80x9cpneumatic hammerxe2x80x9d works in this way by the alternate application of opposite forces by directing pressure fluid alternately into opposite chambers of a pneumatic cylinder with a flutter valve.
If the moving element has significant inertia, against which the external xe2x80x9cstatorxe2x80x9d of the machine is designed to experience a reaction, a controlled vibration can be applied: this may be used for example in association with earth-moving machinery. As a third example, the motion of the an unrestricted piston might be used to vary the total enclosed volume and/or pressure of a fluid system to which the piston chamber is connected. Such a machine might be used as a fluid pump, to dispense metered fluid, to vary the pressure in a sensory pad or to form part of a sensitive and precise fluid pressure suspension system.
Examples of such devices are disclosed in patent specifications U.S. Pat. No. 4,965,864, GB 6 452 81 and GB 2 017 420.
Specification U.S. Pat. No. 4,965,864 discloses a pump in which there is a hollow cylindrical stator surrounded by a sequence of coils and an armature which acts as the pump piston. The armature is a series of magnets arranged to produce a succession of radially oriented magnetic fields of the same sense. The coils are energised sequentially to cause the armature to oscillate within the stator and perform the pumping action.
Specification GB 6 452 81 discloses a solenoid device which works on the variable reluctance principle. The device consists of a plurality of sequential coils surrounding a hollow cylindrical chamber. A magnetic armature has an axial length such that it overlaps two adjacent coils. Successive energising of the two adjacent coils overlapped by the armature causes a linear motion of the armature, either in a single direction or
Specification GB 6 452 81 discloses a solenoid device which works on the variable reluctance principle. The device consists of a plurality of sequential coils surrounding a hollow cylindrical chamber. A magnetic armature has an axial length such that it overlaps two adjacent coils. Successive energising of the two adjacent coils overlapped by the armature causes a linear motion of the armature, either in a single direction or in a reciprocating motion, as desired.
Specification GB 2 017 420 again discloses a electromagnetic pump which has an armature which acts as an piston and which is caused to reciprocate within a stator chamber by electromagnetic interaction between the armature and a winding surrounding the stator and which produces a reciprocating magnetic field which causes corresponding movement in the armature.
In this case the armature is made of an electrically conducting but non-magnetic material such as aluminum and the device acts, inefficiently, as a reciprocating linear induction motor.
The present invention seeks to provide a linear actuator comprising a stator and an armature in which the armature is not attached to an output member which protrudes from the stator, such as the actuator arm or piston rod of known such actuators. That is to say, the armature of this invention is a piston that is free (or substantially free) to move within the stator volume. As used in this specification the term xe2x80x9cfree pistonxe2x80x9d will be understood to mean a member movable within a working volume without having a force-transmitting member such as a piston rod.
According to one aspect of the present invention, therefore, the present invention provides electromagnetic apparatus for producing an axially directed force comprising a stator having a plurality of axially spaced coils around a working volume within which an armature in the form of a free piston is axially displaceable, the armature having means for producing a magnetic field a substantially radially directed part of which intersects at least some of the coils of the stator such that a resultant axially directed force is generated when a current flows in the coils.
Preferably the physical parameters of the device are so chosen that the electrical coils may be connected to an electronic drive unit for controlling the phase, frequency or amplitude of the current in the coils so as to cause the desired axially-directed electromagnetic force to be created between the armature and the stator.
In one embodiment the armature and the stator are of circular cross section. This makes it easy for the outer surface of the armature to be sealed with respect to the internal surface of the stator, so as to prevent or at least restrict fluid flow between the volumes on either side of the piston.
Means may be provided for sensing the position of the piston so as to optimise the configuration of the currents supplied to the coils by the electronic drive unit.
The electronic drive unit may also be arranged to produce a signal representative of the current supplied to the actuator, the integral of such signal being used to control the pressure of a gas supply to at least to one side of a sealed armature.